The present invention relates, in general, to high frequency field effect transistors (FETs) and more particularly to an improved field effect travelling wave transistor monolithically integrated into a coplanar waveguide for amplification of microwave frequencies.
The use of extremely high frequencies, in the microwave range and above, in amplifiers, oscillators, and like circuits, both analog and digital, has been seriously limited by the poor performance of semiconductor devices such as field effect transistors. Such limitations in performance are due in large measure to the internal impedances, or "parasitics", associated with FETs at high frequencies, and numerous attempts have been made to design such devices in a way that will reduce these impedances.
Further difficulties have been encountered in attempting to provide input and output connections to FET amplifiers at microwave frequencies, for the connection points to the amplifier produce "lumped" impedances which are parasitic and which degrade the high frequency performance of the device. Thus, a lumped impedance at the connection between an input or an output line and the FET device produces undesirable reflections at microwave frequencies and reduces the frequency at which the device can be operated.
An early example of the use of FET devices in high frequency travelling wave amplifiers is U.S. Pat. No. 3,378,738 to George W. McIver, wherein an insulated gate FET is described. In that device, the gate and drain, which serve as input and output transmission lines, respectively, are located on a substrate with the gate overlying the source and being insulated therefrom. U.S. Pat. No. 4,141,021 to David R. Decker is a more recent patent directed to a field effect transistor device wherein the gate and source electrodes are on opposite faces of the active layer, and thus on opposite sides of the channel, to reduce parasitic impedances and to increase the frequency of operation. Other patents, such as U.S. Pat. No. 4,249,190 to Alfred Y. Cho, U.S. Pat. No. 4,129,879 to Tantraporn, U.S. Pat. No. 4,236,166 to Chao et al, and U.S. Pat. No. 2,985,805 to Nelson, also suggest the location of components of the transistor on opposite sides of a substrate in order to reduce parasitics. However, all of these devices are limited in performance at high frequencies by a relatively low incremental transconductance per unit width, and by the continued presence of significant parasitic impedances.
Many of the problems of parasitic impedance limitations in high frequency amplification were solved in a recent FET design developed by the present inventors and another, wherein the device was fabricated to include a semiconductor channel region with a source and a gate located on opposite faces of the channel. The source was of an effective length substantially less than that of the gate, and was located substantially symmetrically with respect to the gate. Two separate drains were located at opposite ends of the channel region and were parallel to each other. The incremental transconductance of the device per unit width was approximately twice that of a single conventional FET of similar design. Since transconductance has a significant effect on the high frequency performance of FETs, the device was capable of greatly improved high frequency operation.
In the preferred form of the above-referenced recent design, the source was formed as a buried semi-conductor region of a selected conductivity type within a non-conductive or semi-insulating substrate. The channel region was formed over the semi-insulating substrate, then drain regions of the same conductivity type as the source were formed at the ends of the channel, and gate and drain electrode metal was deposited on the upper face of the device. Contact with the source region was made by forming an opening in the substrate on the opposite face with respect to the gates and drains. A metalized layer making contact with the source also formed a ground plane. With this arrangement, it was found that the source resistance and inductance were practically eliminated, thereby contributing significantly to the high frequency performance of the device.
Although numerous improvements in operation and response characteristics were achieved by the foregoing design, it presented serious difficulties in fabrication, for construction of such a device requires precision work, including precise alignment of the lithography on opposite sides of a substrate. Techniques for accomplishing this with the precision required to fabricate such devices at reasonable cost do not exist.